Response to K. Caldeira’s E-Letter

Nowhere in our Report (“Ocean warming slows coral growth in the central Red Sea,” N. E. Cantin et al., 16 July 2010, p. 322) do we claim that Red Sea corals are “not sensitive to,” or are not threatened by ocean acidification. On the contrary, we agree that ocean acidification may pose a serious threat to a wide range of marine calcifying organisms during this century. Furthermore, we expect that the impact of ocean acidification on Diploastrea heliopora calcification could be exacerbated, because these corals have already been subjected to chronic thermal stress over the past decade. However, the available data do not suggest that ocean acidification contributed to the abrupt decline in D. heliopora calcification in the Red Sea since 1998. We base this conclusion on two factors: first, there is no evidence for a significant decline in the aragonite saturation state (Ωarg) in the central Red Sea in the past 30 years (the period for which historical data are available), and second, the striking quantitative agreement in the coral calcification response to positive sea surface temperature (SST) anomalies in 1940 through 1941, and in 2008.

Using the “MEROU II” data from 1982 (1) and data from stations we occupied in 2008 on cruise 449-6 of R/V Oceanus, we calculated Ωarg from observed temperature, salinity, alkalinity, and total dissolved inorganic carbon (DIC) data in the top 30 m (2-4). The average Ωarg for the OC-449-6 (October 2008) samples (4.37) was only 0.04 units lower than the average Ωarg from the MEROU II station 155 (September 1982) samples (4.41). Given that this observed change may be influenced by spatial and temporal variability in Red Sea water chemistry, we also calculated a conservative predicted change in Red Sea Ωarg by simply holding temperature, salinity, and alkalinity constant at the MEROU II values, and increasing CO2 partial pressure (pCO2) (seawater) by 45 parts per million by volume, to match the increase in atmospheric pCO2 over this period [for example, (5)]. This resulted in a predicted Ωarg decrease of 0.27 by 2008 relative to 1982. Had we included the impact of increasing SST over the past 30 years, the calculated decrease in Ωarg from 1982 to 2008 would have been even smaller. This change in Ωarg (-0.04 to -0.27) is quite small, and the predicted coral calcification response to this change, based on experimental data (6, 7), is negligible.

The sensitivity of the coral calcification response to the 1940 through 1941 warm event (-16 ± 2% per °C) is equivalent to that exhibited by D. heliopora to elevated SSTs more than 4 decades later, (-13 ± 3% per °C). This striking agreement suggests that calcification declined due to elevated SST. If D. heliopora calcification had been “adversely affected by ocean acidification” between 1998 and 2008, we would expect the change in calcification to have been amplified relative to that in 1940; this was not observed.

Ocean acidification may affect coral calcification in the Red Sea as atmospheric CO2 continues to rise. Continued experimental studies and field-based observations of both seawater chemistry and coral calcification will certainly strengthen our understanding of how calcifying organisms and coral reef ecosystems will respond to these impacts. However, the data suggest that ocean acidification did not play a major role in the decline in Red Sea D. heliopora calcification over the past decade.

Neal Cantin, Anne Cohen, Dan McCorkle

Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.


1. A. Papaud, A. Poisson, J. Mar. Res. 44, 385 (1986).

2. D. W. R. Lewis, W. R. Wallace, MS Excel Program Developed for CO2 System Calculations (Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge Tennessee, 1998).

3. C. Mehrbach, C. H. Culberson, J. E. Hawley, R. M. Pytkowicz, Limnol. Oceanogr. 18, 897 (1973).

4. A. G. Dickson, F. J. Millero, Deep-Sea Res. 34, 1733 (1987).

5. NOAA Earth System Research Laboratory, Global Monitoring Division, Trends in Atmospheric Carbon Dioxide (

6. C. Langdon, M. J. Atkinson, J. Geophys. Res. 110, C09S07 (2005).

7. K. Schneider, J. Erez, Limnol. Oceanogr. 51, 1284 (2006).

Neal Cantin, Anne Cohen & Dan McCorkle, 14 February 2011. Article.

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